1. Technical Field of Invention
The present invention relates generally to a braking force controlling system for automotive vehicles, and more particularly to an antiskid brake system designed to regulate the brake fluid pressure developing a braking force acting on each wheel according to a wheel slippage during braking so as to bring the slip ratio into a suitable range.
2. Description of Related Art
Antiskid brake systems which determine a slip ratio of each wheel during braking and control the brake fluid pressure acting on each wheel so as to bring each wheel into an allowable range of slip ratio (usually, 10% to 20%) within which the vehicle can be braked in safety are well known.
In such antiskid brake systems, when the acceleration of one of the wheels shows a negative value when a slip ratio thereof exceeds a target value (e.g., 15%) during braking, the brake fluid pressure applied to that wheel is reduced. If the acceleration shows another value, the brake fluid pressure is held as is. When the slip ratio becomes less than the target value, the brake fluid pressure is increased. Instead of such direct comparison between an actual slip ratio with a target value, a physical quantity corresponding to the slip ratio such as wheel speed may be used. Specifically, an actual wheel speed may be determined and compared with a target wheel speed corresponding to a target slip ratio.
The above conventional antiskid brake systems, however, have a drawback. A change in wheel speed caused by a change in tire-road friction or adhesion .mu. during braking, adversely affects the comparison between the actual slip ratio and the target slip ratio or between the actual wheel speed and the target wheel speed, thereby resulting in instability of antiskid brake control.
The instability of antiskid brake control also results from a change in wheel speed due to irregularities of a road surface, turning of the vehicle, or a difference in diameter between tires.